A standard nozzle or die for an extruder has an outer part and an inner or core part together defining an annular passage. The parts form an inlet passage connected to the extruder and webs extending between the parts support the core on the outer part and define branch passages that connect the inlet passage to the annular passage. Such a die is used to extrude, tubes or sleeves and its outlet can form the workpiece directly or be connected to a further die from which the workpiece issues.
In the known systems the webs have an effect on the finished workpiece that can be determined by microscopic examination of it. The result is that the workpiece is not perfectly uniform and in fact is normally measurably weaker in the regions where the resin flow was interrupted by the webs of the die. Even when as described in German patent 3,332,262 (filed 07 Sept. 1983 by H. Reifenhauser and P. Reitemeyer) the branch passages are formed so that they decrease and then increase in flow cross section in the direction of flow of the resin through them, there are some improvements in the uniformity of the finished product, but there still is room for improvement.